Trailers are typically hitched at the very rear of a towing vehicle for convenience. However that connection point is subjected to significant and essentially instantaneous changes in the direction and the magnitude of the local velocity vector whenever the towing vehicle is steered to a new path radius. The sudden change in direction of the hitch point, either in pitch or sway or both, is serious since it is out of sync with the actual changes in direction of the towing vehicle. The result is a transient phase where the towed vehicle first changes in the opposite direction from the towing vehicle and then must change its direction at least two more times to merge with the new path of the towing vehicle. Thus the towed vehicle goes through a zigzag motion in order to move over and get in-line with a new towing path. Dynamically the adjustment can take multiple cycles and easily lead to developing a swerve or swaying motion, which, in turn, can lead to loss of control and/or an accident.
Historically trailers for automotive vehicles grew out of approaches used for wagons pulled by animals which were influenced obviously by the need to reduce or virtually eliminate the down load on the pulling animals. Thus the typical heavily loaded wagon utilized an axle at its front and rear with the front axle steered by tracking the animal's path.
That arrangement worked well for the early versions of motor vehicles and was commonly used in the 1940's and was effective for the light motor vehicles of that time. However the steering on those trailers was not very well implemented and they typically tended to sway even at slow city speeds and were not used for highway type travel; and were soon phased out. Short dolly hitches for a rear bumper connection were also tried as a means to alleviate the down load on those early cars. However they were pretty much a disaster and immediately discontinued because the dolly's short wheel base was very sensitive to the rear mounted hitch's zigzag motion.
The popular equalizer hitch moves the down load of the trailer forward by applying a spring-loaded moment to the towing vehicle. Thus by relieving the de-stabilizing up load on the towing vehicle's front axle it tends to preserve the towing vehicle's handing to the extent that an experienced driver can tolerate a properly balance towed trailer.
Hitch manufacturers have attempted to reduce the effects of the swaying motion of rear mounted hitched trailers with various energy absorbing devices at the connection to the towing vehicle. However the zigzag path of the rear-mounted hitch is not eliminated for a single point hinged connection aft of the towing vehicle's rear axle and with a friction energy absorbing device it is difficult to take out enough energy without deteriorating the maneuvering ability of the vehicles.
U.S. Pat. No. 4,106,794 (“the '794 patent”), the entirety of which is incorporated herein by reference, is one attempt to control pitch and sway. In one embodiment, a multi-bar linkage towing system attached to the rear of a towing vehicle aft of the rear axle is designed to emulate a fifth wheel attachment. The linkage system is designed for use with non-specialized towing vehicles, such as passenger cars, where a traditional fifth wheel system cannot be implemented. The linkage system comprises multiple tow bars positioned vertically and laterally of each other which, due to their orientation, create a virtual pivot point at the center of the rear axle of the towing vehicle.
This linkage shifts the trailer loads applied at the rear bumper of the towing vehicle forward mechanically to its rear axle without the use of spring bars as is used in the present popular equalizer hitches and thus it is not dependent on preloaded springs or their deflection, only the applied load. In addition, the mechanism of the '794 patent shifts the lateral trailer side loads forward in the same manner and this also significantly stabilizes the towing and towed vehicle combination. The '794 patent mechanism simulates fifth-wheel towing, moves both the vertical and lateral loads forward with its mechanical linkage and accomplishes these advantages without the use of adjustable springs. The difficulty with this mechanism is that it essentially simulates a fifth wheel hitch implementing fifth-wheel towing benefits but with some of the undesirable fifth-wheel disadvantages such as cutting the corner on sharp turns. One of the negative characteristics of present typical fifth-wheel hitch towed vehicles is that, the towed vehicle turns at a much sharper radius than the towing vehicle because of the typical location of the towed vehicle relative to the towing vehicle's rear axle. Thus the typical fifth-wheel towed vehicle significantly cuts sharp corners typical of city intersections.
A corner-cutting solution now provided by some fifth-wheel hitch systems is to move the fifth-wheel hitch point aft for city driving and then move it back forward for the higher speed roadways. The significance of this is to trade-off the stable fifth-wheel connection location for less stability and more corner clearance where the typical speed is much slower. This is fairly acceptable considering that safe city cornering speeds for these types of trailer vehicles are much slower than even normal city traffic. However, of critical consideration here is that typical speeds for city driving in many cases are in the 30 to 50 miles per hour range where lateral stability should be provided. Thus the location-shifting should be done essentially for each corner or at least moved back to its stable position when the expected speeds increase.
Another example of efforts to emulate a fifth-wheel type trailer connection is described in U.S. Pat. Nos. 7,497,457, 7,823,902 and 8,042,825. The towing system described in these patents is designed for use with gooseneck-type trailers that utilize fifth wheel-type attachments. The system includes an accessory towing apparatus, which is attached to the towing vehicle and effectively extends the towing vehicle's wheelbase for purposes of enhancing stability and control. The accessory towing apparatus comprises a chassis with two wheels and two linkage arms extending forward from the accessory towing apparatus to engage the towing vehicle. The axle assembly is comprised of ground engaging wheels having a fixed alignment or a dynamically induced alignment. Thus they are essentially castor-mounted wheels, which can in one case be locked in alignment or in a second case set free to swivel dynamically. The first case is proposed for use at high speeds with the wheels fixed and allows only a slight swivel motion. Thus the towing vehicle is restricted in its lateral motion and this supposedly stabilizes the towing and towed vehicle combination. At slow speeds the axle assembly enables the wheels to pivot up to 31 degrees thus permitting the vehicles wheel's to castor and be maneuvered. The concept is to lock the castored wheels for stability and to unlock them to allow the towing vehicle to maneuver.
The problem foreseen with such an approach is that a castored wheel steers itself dynamically because it has no lateral resistance to swiveling and thus it tracks dynamically in the direction in which it is pushed. Thus a freely castored wheel cannot provide any lateral support to improve the lateral stability. Whenever these support wheels are un-locked, to swivel even a small amount, there is no lateral stability for the extended hitch position and the towing vehicle becomes very unstable, much more so than if the hitch were at the rear bumper. Plus any attempt to maneuver with the swivels locked will be aggravated by the requirement that to maneuver laterally for the locked case one or more of the three axles on the ground will have to slide laterally. The result is the handling and stability of the so coupled vehicles at any given time will depend on which axles are sliding and to what degree. Thus both the locked and unlocked cases will present stability and handling risks.